DOC AIRBUS | AMM A320FTRAFFIC COLLISION AVOIDANCE SYSTEM (TCAS) - DESCRIPTION AND OPERATION
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1. General
A. Presentation
The TCAS II (Traffic Collision Avoidance System) is a system whose function is to detect and display aircraft in the immediate vicinity and to provide the flight crew with indications to avoid these intruders by changing the flight path in the vertical plane only.
TCAS change 7 has been mandatory in Europe since year 2000.
TCAS Change 7.1 has been implemented as an improvement of the collision avoidance algorithm and alerting.
The TCAS periodically interrogates their transponders, computes their trajectories and constantly determines their potential threat. The acquisition of their signals is achieved by means of two transmit/receive antennas, one located on the underside of the fuselage and the other on the top.
The system can establish individualized communications with each aircraft through ATC/Mode S transponders, thus permitting operation in dense traffic areas while avoiding an overload of radio-electric transmissions that would result from a general all-intruder response.
The TCAS II system is designed to provide the air traffic control system with an additional possibility. It usually operates independently but may be controlled from ground stations.
The TCAS has the capability to communicate with ATCRBS (Air Traffic Control Radar Beacon System) ground stations equipped with the Mode S system to indicate to them the vertical maneuver orders presented to the aircraft pilot. This information can facilitate the task of the ground station controller who, in turn, can modify the TCAS operating mode and cancel the avoidance orders if he deems it necessary for safety.
The system maintains surveillance within a sphere determined by the transmit power and receiver sensitivity of the TCAS computer. The area in which a threat is imminent depends on the speed and path of the own A/C and the threat A/C.
There is an area defined as TAU within the surveillance arc which represents the minimum time the flight crew needs to discern a collision threat and take evasive action.
The TCAS detection capability covers an area of 35-40 NM in range and plus or minus 9900 ft in altitude but display range is authorized up to plus or minus 2700 ft in altitude.
The TCAS II (Traffic Collision Avoidance System) is a system whose function is to detect and display aircraft in the immediate vicinity and to provide the flight crew with indications to avoid these intruders by changing the flight path in the vertical plane only.
TCAS change 7 has been mandatory in Europe since year 2000.
TCAS Change 7.1 has been implemented as an improvement of the collision avoidance algorithm and alerting.
The TCAS periodically interrogates their transponders, computes their trajectories and constantly determines their potential threat. The acquisition of their signals is achieved by means of two transmit/receive antennas, one located on the underside of the fuselage and the other on the top.
The system can establish individualized communications with each aircraft through ATC/Mode S transponders, thus permitting operation in dense traffic areas while avoiding an overload of radio-electric transmissions that would result from a general all-intruder response.
The TCAS II system is designed to provide the air traffic control system with an additional possibility. It usually operates independently but may be controlled from ground stations.
The TCAS has the capability to communicate with ATCRBS (Air Traffic Control Radar Beacon System) ground stations equipped with the Mode S system to indicate to them the vertical maneuver orders presented to the aircraft pilot. This information can facilitate the task of the ground station controller who, in turn, can modify the TCAS operating mode and cancel the avoidance orders if he deems it necessary for safety.
The system maintains surveillance within a sphere determined by the transmit power and receiver sensitivity of the TCAS computer. The area in which a threat is imminent depends on the speed and path of the own A/C and the threat A/C.
There is an area defined as TAU within the surveillance arc which represents the minimum time the flight crew needs to discern a collision threat and take evasive action.
The TCAS detection capability covers an area of 35-40 NM in range and plus or minus 9900 ft in altitude but display range is authorized up to plus or minus 2700 ft in altitude.
NOTE: The ABOVE/BELOW function on the ATC/TCAS control unit is used to increase the display range above 1200 ft and below -1200 ft.
NOTE: The description of the system conforms to RTCA/DO-185A Change 7.0.
B. Principle
When an aircraft is airborne, its TCAS periodically transmits interrogation signals for all ATCRBS and Mode S transponder-equipped aircraft in the vicinity. These interrogations are received by the ATCRBS ground stations and by the transponders of the other aircraft.
In response to these interrogations, the transponders of nearby aircraft return signals containing their altitude value. The TCAS computes the range between the two aircraft by measuring the elapsed time between transmission of the interrogation and reception of the reply.
The altitude, altitude rate, range and range rate are determined by a periodic tracking of these exchanges and the data are used for intruder threat assessment.
Each threat is treated individually but the TCAS determines the best collision avoidance possibility with respect to all aircraft in its vicinity, while establishing maneuver coordination with the other TCAS-equipped aircraft. The optimum maneuver is the one that ensures an adequate separation of trajectories with a minimum vertical trend variation.
When an aircraft is airborne, its TCAS periodically transmits interrogation signals for all ATCRBS and Mode S transponder-equipped aircraft in the vicinity. These interrogations are received by the ATCRBS ground stations and by the transponders of the other aircraft.
In response to these interrogations, the transponders of nearby aircraft return signals containing their altitude value. The TCAS computes the range between the two aircraft by measuring the elapsed time between transmission of the interrogation and reception of the reply.
The altitude, altitude rate, range and range rate are determined by a periodic tracking of these exchanges and the data are used for intruder threat assessment.
Each threat is treated individually but the TCAS determines the best collision avoidance possibility with respect to all aircraft in its vicinity, while establishing maneuver coordination with the other TCAS-equipped aircraft. The optimum maneuver is the one that ensures an adequate separation of trajectories with a minimum vertical trend variation.
C. Advisories
Visual and aural advisories are supplied by the TCAS computer whenever assessment of the relative position of two aircraft reveals a potential collision hazard.
The Traffic Advisories (TA) indicate the position of nearby aircraft which are or may become a threat. Their display alerts the flight crew to the presence of intruders and facilitates their visual acquisition.
The Resolution Advisories (RA) may be divided into two categories:
Visual and aural advisories are supplied by the TCAS computer whenever assessment of the relative position of two aircraft reveals a potential collision hazard.
The Traffic Advisories (TA) indicate the position of nearby aircraft which are or may become a threat. Their display alerts the flight crew to the presence of intruders and facilitates their visual acquisition.
The Resolution Advisories (RA) may be divided into two categories:
- Corrective Advisories that instruct the pilot to deviate from current vertical rate
- Preventive Advisories that instruct the pilot to avoid certain maneuvers.
D. Display
Visual indications are presented on the Electronic Flight Instrument System (EFIS).
The Navigation Display (ND) is used to indicate the situation in the nearby traffic area: a symbol is displayed for each intruder on the image in the ARC or ROSE mode.
The avoidance maneuver indications, if any, are displayed on the vertical speed scale of the Primary Flight Display (PFD) by means of a band of colored sectors showing the vertical speed value to be adopted in order to avoid any risk of collision.
Visual indications are presented on the Electronic Flight Instrument System (EFIS).
The Navigation Display (ND) is used to indicate the situation in the nearby traffic area: a symbol is displayed for each intruder on the image in the ARC or ROSE mode.
The avoidance maneuver indications, if any, are displayed on the vertical speed scale of the Primary Flight Display (PFD) by means of a band of colored sectors showing the vertical speed value to be adopted in order to avoid any risk of collision.
E. Coordination
The avoidance maneuvers initiated by the TCAS could create a conflict situation if directed at another TCAS-equipped aircraft as this aircraft may also take similar evasive action, resulting in an unchanged situation.
To avoid this situation, a communication link between the two aircraft is established via the transponders, exchanging coordination messages.
The first aircraft to detect the other one initiates the communication procedure, indicates the maneuvers it intends to perform and communicates orders to the other aircraft requesting it to maintain its trajectory.
This necessarily involves the use of Mode S transponders, the only equipment of this type possessing the LINK function required for data exchange.
The Mode S transponders provide the capability to transmit a unique address (24 bits) assigned to each aircraft, permitting them to reply individually to other TCAS-equipped aircraft. It can respond to ground station interrogations in Mode A and Mode C and also in Mode S if the stations are suitably equipped.
The ground stations can modify the TCAS operating mode via the transponder link so as to inhibit resolution advisories in certain conditions.
The avoidance maneuvers initiated by the TCAS could create a conflict situation if directed at another TCAS-equipped aircraft as this aircraft may also take similar evasive action, resulting in an unchanged situation.
To avoid this situation, a communication link between the two aircraft is established via the transponders, exchanging coordination messages.
The first aircraft to detect the other one initiates the communication procedure, indicates the maneuvers it intends to perform and communicates orders to the other aircraft requesting it to maintain its trajectory.
This necessarily involves the use of Mode S transponders, the only equipment of this type possessing the LINK function required for data exchange.
The Mode S transponders provide the capability to transmit a unique address (24 bits) assigned to each aircraft, permitting them to reply individually to other TCAS-equipped aircraft. It can respond to ground station interrogations in Mode A and Mode C and also in Mode S if the stations are suitably equipped.
The ground stations can modify the TCAS operating mode via the transponder link so as to inhibit resolution advisories in certain conditions.
2. Component Location
** ON A/C NOT FOR ALL | FIN | FUNCTIONAL DESIGNATION | PANEL | ZONE | ACCESS DOOR | ATA REF |
|---|---|---|---|---|---|
| ** ON A/C NOT FOR ALL | |||||
| 3SH | CTL UNIT-ATC/TCAS | 11VU | 210 | 34-52-12 | |
| 1SG | CMPTR-TCAS | 82VU | 128 | 34-43-34 | |
| 7SG1 | ANTENNA-TCAS, TOP | 230 | 34-43-11 | ||
| 7SG2 | ANTENNA-TCAS, BOTTOM | 130 | 34-43-11 | ||
3. System Description
TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL
TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL
TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL A. Principle
The TCAS II uses the ATC/Mode S transponders to locate and identify the aircraft in the protected airspace which are equipped with ATC transponders. Traffic advisories are displayed on the Navigation Displays (ND). If an aircraft constitutes a risk of collision, the TCAS II computer provides the resolution advisory for collision avoidance in the form of a vertical maneuver which will increase separation between both aircraft; this advisory is displayed on the vertical speed scale of the Primary Flight Display (PFD) and a synthetic voice is transmitted through the cockpit loud speakers.
The TCAS II uses the ATC/Mode S transponders to locate and identify the aircraft in the protected airspace which are equipped with ATC transponders. Traffic advisories are displayed on the Navigation Displays (ND). If an aircraft constitutes a risk of collision, the TCAS II computer provides the resolution advisory for collision avoidance in the form of a vertical maneuver which will increase separation between both aircraft; this advisory is displayed on the vertical speed scale of the Primary Flight Display (PFD) and a synthetic voice is transmitted through the cockpit loud speakers.
B. System Architecture
The TCAS will comprise:
TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL
TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL
The TCAS environment is composed of items closely associated with its operation, such as the transponders and the EFIS display units and peripherals supplying parameters or with maintenance functions.
The TCAS will comprise:
- A TCAS II computer 1SG
- Two directional antennas 7SG1, 7SG2
In addition, the system is linked to the two independent ATC/Mode S systems, each comprising: - A transponder 1SH1 (1SH2)
- Two antennas 7SH1, 7SH3 (7SH2, 7SH4)
- An ATC/TCAS control unit 3SH
- The MCDU and the centralized fault display interface unit (CFDIU) 1TW for test purpose.
The EFIS display units display the TCAS data.
TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL TCAS Block Diagram - Maintenance and Display ** ON A/C NOT FOR ALL C. TCAS Computer
The TCAS computer is the heart of the TCAS II system.
It complies with the dimensional standard in ARINC 600 for 6 MCU form factor.
It is compatible with ARINC 600 forced air cooling.
It ensures two main functions:
The TCAS computer is the heart of the TCAS II system.
It complies with the dimensional standard in ARINC 600 for 6 MCU form factor.
It is compatible with ARINC 600 forced air cooling.
It ensures two main functions:
- A radio-electric transmission reception function in the L-band frequency for intruder acquisition
- A processing function ensuring total operation control: digital, discrete and analog-type interfaces, intruder trajectory computation and tracking, visual and aural alert commands.
D. Directional Antennas
The TCAS has two antennas, one located on the top of the aircraft and the other on the underside of the fuselage. These antennas, of the transmit/receive type, provide azimuth information on aircraft located within the TCAS surveillance range. They consist of four independent elements. In reception, the amplitude of the signals received by each element depends on the direction of the signal source, which permits the relative bearing of the transmitting aircraft to be determined.
The TCAS has two antennas, one located on the top of the aircraft and the other on the underside of the fuselage. These antennas, of the transmit/receive type, provide azimuth information on aircraft located within the TCAS surveillance range. They consist of four independent elements. In reception, the amplitude of the signals received by each element depends on the direction of the signal source, which permits the relative bearing of the transmitting aircraft to be determined.
E. ATC/TCAS Control Unit
The ATC/TCAS control unit common to the ATC transponders and the TCAS, enables the operating modes of these two items of equipment to be selected. Information intended for the TCAS is transmitted via an ARINC 429 bus to the transponders which in turn transmit it to the TCAS computer.
The ATC/TCAS control unit common to the ATC transponders and the TCAS, enables the operating modes of these two items of equipment to be selected. Information intended for the TCAS is transmitted via an ARINC 429 bus to the transponders which in turn transmit it to the TCAS computer.
F. Associated Items
(1) ATC/Mode S transponders
Two ATC/Mode S transponders, one active and the other on standby, are used with their antennas. Each transponder is linked to the TCAS by one high-speed ARINC 429 bus for transmission and another one for reception.
Apart from the specific transponder functions (response to ATC ground station interrogations) they permit communication between the TCAS and a TCAS-equipped detected aircraft.
Two ATC/Mode S transponders, one active and the other on standby, are used with their antennas. Each transponder is linked to the TCAS by one high-speed ARINC 429 bus for transmission and another one for reception.
Apart from the specific transponder functions (response to ATC ground station interrogations) they permit communication between the TCAS and a TCAS-equipped detected aircraft.
(2) Displays
Advisories are displayed by the Electronic Instrument System (EIS) by high speed transmission of ARINC 429 messages on the buses linking the TCAS to the Display Management Computers (DMCs) 1 and 3 on the one hand and to the DMC 2 on the other hand. The Flight Warning Computers (FWCs) connected in parallel on these buses monitor the validity of the information.
The Navigation Displays (NDs) provide indications on the location of intruders in the traffic area. The Primary Flight Displays (PFDs) provide the flight crew with vertical speed correction information to avoid them.
The EWD also presents warning messages.
Advisories are displayed by the Electronic Instrument System (EIS) by high speed transmission of ARINC 429 messages on the buses linking the TCAS to the Display Management Computers (DMCs) 1 and 3 on the one hand and to the DMC 2 on the other hand. The Flight Warning Computers (FWCs) connected in parallel on these buses monitor the validity of the information.
The Navigation Displays (NDs) provide indications on the location of intruders in the traffic area. The Primary Flight Displays (PFDs) provide the flight crew with vertical speed correction information to avoid them.
The EWD also presents warning messages.
(3) ATC/Mode S transponders
Two ATC/Mode S transponders, one active and the other on standby, are used with their antennas. Each transponder is linked to the TCAS by one high-speed ARINC 429 bus for transmission and another one for reception.
Apart from the specific transponder functions (response to ATC ground station interrogations) they permit communication between the TCAS and a TCAS-equipped detected aircraft.
Two ATC/Mode S transponders, one active and the other on standby, are used with their antennas. Each transponder is linked to the TCAS by one high-speed ARINC 429 bus for transmission and another one for reception.
Apart from the specific transponder functions (response to ATC ground station interrogations) they permit communication between the TCAS and a TCAS-equipped detected aircraft.
(4) Displays
Advisories are displayed by the Electronic Instrument System (EIS) by high speed transmission of ARINC 429 messages on the buses linking the TCAS to the Display Management Computers (DMCs) 1 on CAPT side and to the DMC 2 on F/O side. Depending on the EFIS DMC selection, the DMC 3 receives either the TA/RA display bus 1 or the TA/RA display bus 2. The DMC3 is connected to the bus TA/RA2 when the DMC reconfiguration switch is on position F/O/3. The FWCs connected in parallel on these buses monitor the validity of the information.
The Navigation Displays (NDs) provide indications on the location of intruders in the traffic area. The Primary Flight Displays (PFDs) provide the flight crew with vertical speed correction information to avoid them.
The EWD also presents warning messages.
Advisories are displayed by the Electronic Instrument System (EIS) by high speed transmission of ARINC 429 messages on the buses linking the TCAS to the Display Management Computers (DMCs) 1 on CAPT side and to the DMC 2 on F/O side. Depending on the EFIS DMC selection, the DMC 3 receives either the TA/RA display bus 1 or the TA/RA display bus 2. The DMC3 is connected to the bus TA/RA2 when the DMC reconfiguration switch is on position F/O/3. The FWCs connected in parallel on these buses monitor the validity of the information.
The Navigation Displays (NDs) provide indications on the location of intruders in the traffic area. The Primary Flight Displays (PFDs) provide the flight crew with vertical speed correction information to avoid them.
The EWD also presents warning messages.
G. Peripherals
The TCAS receives information from the following equipment:
The TCAS receives information from the following equipment:
(1) Radio altimeter
The radio altitude is an essential parameter for the TCAS. In the 0 to 2500 ft range it permits modulation of system sensitivity and triggering of inhibit orders. For this reason, two radio altimeters, one active and the other in standby, are linked to the TCAS via the low speed ARINC 429 bus.
The radio altitude is an essential parameter for the TCAS. In the 0 to 2500 ft range it permits modulation of system sensitivity and triggering of inhibit orders. For this reason, two radio altimeters, one active and the other in standby, are linked to the TCAS via the low speed ARINC 429 bus.
(2) Air data system
Barometric altitude information fulfils the same functions as radio altitude information but for the range over 2500 ft. The ADIRU, which is not directly linked to the TCAS computer, transmits this information to the computer across the transponders via the ARINC 429 buses.
Barometric altitude information fulfils the same functions as radio altitude information but for the range over 2500 ft. The ADIRU, which is not directly linked to the TCAS computer, transmits this information to the computer across the transponders via the ARINC 429 buses.
(3) Inertial reference system
The TCAS is linked by a high speed ARINC 429 bus to the aircraft ADIRU1 which supplies it with magnetic heading and pitch and roll attitude information.
The TCAS is linked by a high speed ARINC 429 bus to the aircraft ADIRU1 which supplies it with magnetic heading and pitch and roll attitude information.
(4) Landing Gear Control and Interface Unit (LGCIU)
The LGCIU provides the TCAS with the following discrete signals:
The LGCIU provides the TCAS with the following discrete signals:
- ground/flight signal
- landing gear extended signal.
(5) Centralized Fault Display Interface Unit (CFDIU)
The TCAS computer communicates with the Centralized Fault Display System (CFDS) via two low speed ARINC 429 buses.
The TCAS computer communicates with the Centralized Fault Display System (CFDS) via two low speed ARINC 429 buses.
(6) Flight Data Interface Unit (FDIU)
The FDIU receives TCAS data from the DMC.
The following parameters are recorded by the FDIU:
The FDIU receives TCAS data from the DMC.
The following parameters are recorded by the FDIU:
- RA related information (label 270):
* advisory rate to maintain
* combined control
* vertical control
* up advisory
* down advisory - manual sensitivity level (SL 0, 1, 2)
- TA issued or not.
(7) Various discrete signals
Discretes are used for the inhibition of certain advisories by equipment with higher priority than the TCAS:
Discretes are used for the inhibition of certain advisories by equipment with higher priority than the TCAS:
- advisory inhibit by the windshear signal
- advisory inhibit by the stall warning
- advisory inhibits by the ground proximity warning.
(8) Pin programming
Discrete pin program inputs are used:
Discrete pin program inputs are used:
- audio level to set the audio level of the synthetic voice output
- all traffic/threat traffic display at Threat Traffic indicates that intruders are only displayed if a TA or RA is presented
- ground display mode: specifies that the display mode, when the aircraft is on the ground, is the TA mode only
- display intruder limit: indicates the maximum number of intruders that can be displayed (8)
- aircraft altitude limit, configured at 48,000 ft informs the TCAS computer that it must inhibit climb orders above this altitude.
4. Power Supply
The TCAS is supplied with 115VAC, 100 watt power rating. The ATC/TCAS control unit, common to the transponders and the TCAS, is also supplied with 115VAC:
The TCAS is supplied with 115VAC, 100 watt power rating. The ATC/TCAS control unit, common to the transponders and the TCAS, is also supplied with 115VAC:
- The 115VAC BUS1 101XP supplies the TCAS via circuit breaker 4SG
- The 115VAC SHED ESS BUS 801XP supplies the ATC1 system via circuit breaker 5SH1
- The 115VAC BUS2 204XP supplies the ATC2 system via circuit breaker 5SH2.
The ATC/TCAS control unit is energized through the two circuit breakers 5SH1 and 5SH2.
The system is supplied through these circuit breakers.
** ON A/C NOT FOR ALL
5. Component Description
A. TCAS Computer FIN: 1000-SG
(1) Introduction
The computer is divided into four major sections:
The computer is divided into four major sections:
- Traffic/Power Supply Circuit Card Assembly (T/PS CCA)
- System I/O Processor Circuit Card Assembly (SysI/O CCA)
- Radio Frequency Module (RF Module)
- Chassis Assembly (not in the figure)
(2) Traffic/Power Supply Circuit Card Assembly (T/PS CCA)
The computer contains an AC power supply. This power supply provides all modules in the computer.
The CCA also provides all ACAS surveillance functionality. The Traffic/Power Supply Circuit Card Assembly (T/PS CCA) is interfaced with the SysI/O CCA and to the Transmitter/Receiver (TRx). All these interfaces are linked through the backplane CCA.
The computer contains an AC power supply. This power supply provides all modules in the computer.
The CCA also provides all ACAS surveillance functionality. The Traffic/Power Supply Circuit Card Assembly (T/PS CCA) is interfaced with the SysI/O CCA and to the Transmitter/Receiver (TRx). All these interfaces are linked through the backplane CCA.
(3) System I/O Processor Circuit Card Assembly (SysI/O CCA)
The System I/O Processor Circuit Card Assembly (SysI/O CCA) is intended to handle all primary data I/O interfaces between the aircraft and the computer software functions. The SysI/O CCA is intended to contain aircraft specific interfaces.
The System I/O Processor Circuit Card Assembly (SysI/O CCA) is intended to handle all primary data I/O interfaces between the aircraft and the computer software functions. The SysI/O CCA is intended to contain aircraft specific interfaces.
(4) Radio Frequency Module (RF Module)
The Transmitter/Receiver (TRx) Circuit Card Assembly (CCA) provides the hardware circuits for all transmit and receive functions. The TRx module contains the four transmit and receive channels. Each channel is connected via switches to one of the four elements of the top or bottom antenna.
The Transmitter/Receiver (TRx) Circuit Card Assembly (CCA) provides the hardware circuits for all transmit and receive functions. The TRx module contains the four transmit and receive channels. Each channel is connected via switches to one of the four elements of the top or bottom antenna.
B. ATC/TCAS Control Unit FIN: 3-SH
For detailed description, Ref. 34-52-00, Para. 5.B.
For detailed description, Ref. 34-52-00, Para. 5.B.
C. TCAS Antennas FIN: 7-SG-1 FIN: 7-SG-2
The directional antenna is composed of four passive vertically-polarized elements. This high-strength composite antenna is provided with a curved base (110 in), eight fuselage mounting screws and four color-coded connectors used to coaxially connect the four antenna elements to the TCAS computer.
A teflon gasket is located between the fuselage and the base of the antenna, in order to facilitate the removal of the antenna.
An O-ring is provided to seal the antenna to the fuselage. The antenna is used to receive and provide directional information for 1090 MHz Mode S squitters, Mode S and Air Traffic Control Radar Beacon System (ATCRBS) replies. Proper phasing of the four antenna elements enables omni or directional transmission of 1030 MHz broadcast or coordination messages and ATCRBS or Mode S interrogations.
The directional antenna is composed of four passive vertically-polarized elements. This high-strength composite antenna is provided with a curved base (110 in), eight fuselage mounting screws and four color-coded connectors used to coaxially connect the four antenna elements to the TCAS computer.
A teflon gasket is located between the fuselage and the base of the antenna, in order to facilitate the removal of the antenna.
An O-ring is provided to seal the antenna to the fuselage. The antenna is used to receive and provide directional information for 1090 MHz Mode S squitters, Mode S and Air Traffic Control Radar Beacon System (ATCRBS) replies. Proper phasing of the four antenna elements enables omni or directional transmission of 1030 MHz broadcast or coordination messages and ATCRBS or Mode S interrogations.
A. Intruder Detection
The TCAS detects A/C equipped with Mode S transponders by listening for squitter transmissions. Mode S transponders announce their presence by transmitting squitter messages once every second. The TCAS also detects A/C equipped with transponders that do not reply to Mode S interrogations but do reply to Mode C interrogations. The TCAS must actively search for Mode C equipped intruder aircraft because Mode C transponders do not transmit squitter messages. Once the presence of a Mode C intruder is confirmed, it is tracked by the TCAS. The TCAS is capable of tracking up to a combined total of 30 Mode S and Mode C intruders.
Tracking is performed by repetitive TCAS interrogations in Mode S and Mode C format.
The TCAS detects A/C equipped with Mode S transponders by listening for squitter transmissions. Mode S transponders announce their presence by transmitting squitter messages once every second. The TCAS also detects A/C equipped with transponders that do not reply to Mode S interrogations but do reply to Mode C interrogations. The TCAS must actively search for Mode C equipped intruder aircraft because Mode C transponders do not transmit squitter messages. Once the presence of a Mode C intruder is confirmed, it is tracked by the TCAS. The TCAS is capable of tracking up to a combined total of 30 Mode S and Mode C intruders.
Tracking is performed by repetitive TCAS interrogations in Mode S and Mode C format.
(1) Interrogation of aircraft equipped with Mode A or Mode C transponders
With respect to aircraft equipped with Mode A or Mode C transponders, the TCAS is active and transmits Mode C only all-call interrogations (P1, P3 and P4 pulses). The code is similar to the one used by the Mode A and Mode C ground stations. The P4 pulse informs those Mode S transponders that this interrogation is not addressed to them
The nominal time interval between two interrogations is one second. But, to limit radio-electric interference in dense traffic areas, each interrogation consists of a series of interrogations of increasing strength to reach more remote aircraft (whisper-shout) with 1 ms time periods inside the series.
The first transmission consists of relatively low power P1, P3 and P4 pulses only.
Therefore, only the nearest aircraft will receive and reply to these interrogations.
Then an S1 pulse is also transmitted. This pulse is at a lower amplitude, causing the close-in aircraft to interpret this as a side lobe from the transmitting station, requiring no reply. The purpose of the whisper-shout sequence is to reduce the number of aircraft replying to any one interrogation, thus limiting interference.
With respect to aircraft equipped with Mode A or Mode C transponders, the TCAS is active and transmits Mode C only all-call interrogations (P1, P3 and P4 pulses). The code is similar to the one used by the Mode A and Mode C ground stations. The P4 pulse informs those Mode S transponders that this interrogation is not addressed to them
The nominal time interval between two interrogations is one second. But, to limit radio-electric interference in dense traffic areas, each interrogation consists of a series of interrogations of increasing strength to reach more remote aircraft (whisper-shout) with 1 ms time periods inside the series.
The first transmission consists of relatively low power P1, P3 and P4 pulses only.
Therefore, only the nearest aircraft will receive and reply to these interrogations.
Then an S1 pulse is also transmitted. This pulse is at a lower amplitude, causing the close-in aircraft to interpret this as a side lobe from the transmitting station, requiring no reply. The purpose of the whisper-shout sequence is to reduce the number of aircraft replying to any one interrogation, thus limiting interference.
(2) Replies of aircraft equipped with Mode A or Mode C transponders
Aircraft equipped with Mode C transponders reply by transmitting their altitude, octal encoded in four digits ABCD, with a value of 100 ft for the LSB in the ATCRBS format.
For aircraft of which the Mode A and C transponders do not have the altitude report, the intruder presentation on the ND is limited to a display of its position in range and bearing.
Aircraft equipped with Mode A transponders only cannot be detected by the TCAS.
Aircraft equipped with Mode C transponders reply by transmitting their altitude, octal encoded in four digits ABCD, with a value of 100 ft for the LSB in the ATCRBS format.
For aircraft of which the Mode A and C transponders do not have the altitude report, the intruder presentation on the ND is limited to a display of its position in range and bearing.
Aircraft equipped with Mode A transponders only cannot be detected by the TCAS.
(3) Interrogation of aircraft equipped with Mode S transponders
(a) Transmission coding
The TCAS uses the Mode S function for certain identification of intruders as a 24-bit address is definitively assigned to each aircraft by air traffic control.
The interrogation comprises three pulses: P1, P2 and P6. P2 level is equal to or greater than the P1 level, which is the no-reply condition for the aircraft equipped with Mode A or C transponders. Therefore, only Mode S transponders reply to the interrogation.
The useful information is contained in P6 divided into 56 or 112 chips.
A chip is an unmodulated interval of 0.25 microseconds, preceded by possible phase reversals.
The message formats contain a number of bits permitting a more complete and diversified information exchange than in Mode C.
There are two distinct message formats:
The TCAS uses the Mode S function for certain identification of intruders as a 24-bit address is definitively assigned to each aircraft by air traffic control.
The interrogation comprises three pulses: P1, P2 and P6. P2 level is equal to or greater than the P1 level, which is the no-reply condition for the aircraft equipped with Mode A or C transponders. Therefore, only Mode S transponders reply to the interrogation.
The useful information is contained in P6 divided into 56 or 112 chips.
A chip is an unmodulated interval of 0.25 microseconds, preceded by possible phase reversals.
The message formats contain a number of bits permitting a more complete and diversified information exchange than in Mode C.
There are two distinct message formats:
- all Mode S interrogations (UPLINK format) are binary differential phase shift keying (DPSK) signals
- Mode S replies (DOWNLINK format) are formed by pulse position modulation (PPM) encoding the reply data.
The Mode S reply is preceded by a preamble containing four pulses of specific duration and intervals intended to guarantee received message validity. Any messages whose preamble is not in complete conformity with the model are rejected by the TCAS. This information is encoded in PPM mode with, for each bit, a logic level one if the first half of the interval is at 1 and a logic level zero if it is at zero.
(b) Squitters
The Mode S transponder participates actively in its own detection by transmitting signals, at one second intervals, intended to inform nearby aircraft of its presence.
This transmission, called squitter transmission, consists of a Format DF = 11 message, containing the Mode S 24-bit address assigned to the aircraft, whereas all the bits of the message PI field at zero indicates a squitter.
The Mode S transponder participates actively in its own detection by transmitting signals, at one second intervals, intended to inform nearby aircraft of its presence.
This transmission, called squitter transmission, consists of a Format DF = 11 message, containing the Mode S 24-bit address assigned to the aircraft, whereas all the bits of the message PI field at zero indicates a squitter.
(4) Mode S communication messages
The Mode S ATC communications system supporting personalized exchanges between ATC ground stations and Mode S transponder-equipped aircraft comprises:
DEFINITION OF UPLINK FORMAT MESSAGE FIELDS
DEFINITION OF DOWNLINK FORMAT MESSAGE FIELDS
The Mode S ATC communications system supporting personalized exchanges between ATC ground stations and Mode S transponder-equipped aircraft comprises:
- a set of 25 standard messages for station-aircraft uplinks (Uplink format) identified UFxx
- another set of 25 standard messages for downlinks (Downlink format) identified DFxx.
The message may be long or short and contain either 56 or 112 bits.
Each message consists of specific fields with bit combinations that have specific meanings. Two fields, however, have an identical definition for all messages: - the "message-type" field consisting of bits 1 to 5 whose coding translates the format decimal value into binary
- the address field containing the last 24 bits of the message (bits 33 to 56 or bits 89 to 112 depending on whether the message is short or long). These 24 bits contain the Mode S address of the transmitter.
The TCAS uses this bidirectional link capability to communicate with other TCAS equipped aircraft to coordinate avoidance maneuvers. It may also dialog with ground stations: these stations have the possibility of monitoring and modifying its action.
In the Mode S message set, the TCAS uses only six UF-type messages and seven DF-type messages. These messages are: - UF0, UF4, UF5, UF16, UF20, UF21
- DF0, DF4, DF5, DF11, DF16, DF20, DF21.
The following two figures give the list of messages used by the TCAS for communications with other aircraft and with ground stations.
They are followed by the definition of the fields used in the messages.
NOTE: The TCAS only uses short messages (56 bits).
DEFINITION OF UPLINK FORMAT MESSAGE FIELDS
| ------------------------------------------------------------------------------- |
| ! DESIGNATOR ! FIELD ! INDICATION ! |
| !-------------!----------------!----------------------------------------------! |
| ! AP ! ADDRESS PARITY ! Coded address with parity check ! |
| ! AQ ! ACQUISITION ! Indicates if it is an interrogation message; ! |
| ! ! ! 1 = interrogation ! |
| ! DI ! DESIGNATOR ! Specifies type of information contained in SD! |
| ! ! IDENT ! field ! |
| ! MA ! MESSAGE ! Used by ground station to transmit a TCAS SL ! |
| ! ! Comm-A ! command to a TCAS-equipped aircraft ! |
| ! MU ! MESSAGE ! Used by TCAS to transmit to other aircraft RA! |
| ! ! Comm-U ! coordination information (under fields UDS, ! |
| ! ! ! MTB, CVC, VRC, CHC, HRC, HSB, VSB) ! |
| ! PC ! PROTOCOL ! Operating commands to the transponder ! |
| ! RL ! REPLY LENGTH ! Indicates if message is short (0) or long (1)! |
| ! RR ! REPLY REQUEST ! Length and content of reply information ! |
| ! ! ! requested by the interrogator ! |
| ! SD ! SPECIAL ! Contains control codes affecting the ! |
| ! ! DESIGNATOR ! transponder protocol ! |
| ------------------------------------------------------------------------------- |
DEFINITION OF DOWNLINK FORMAT MESSAGE FIELDS
| ------------------------------------------------------------------------------- |
| ! DESIGNATOR ! FIELD ! INDICATION ! |
| !-------------!----------------!----------------------------------------------! |
| ! AA ! ADDRESS ! Mode S address in the clear in 24 bits ! |
| ! ! ANNOUNCED ! ! |
| ! AC ! ALTITUDE CODE ! Information indicating aircraft altitude ! |
| ! AP ! ADDRESS PARITY ! Coded address with parity check ! |
| ! CA ! CAPABILITY ! Transponder capability ! |
| ! DR ! DOWNLINK ! Requests extraction of downlink message by ! |
| ! ! REQUEST ! the interrogator (existing RA) ! |
| ! FS ! FLIGHT STATUS ! Flight status of the aircraft: ground, ! |
| ! ! ! flight, alert, SPI ! |
| ! ID ! IDENTIFICATION ! Contains the Mode A identification code ! |
| ! ! CODE ! ! |
| ! MB ! MESSAGE ! Indicates Advisory content to the ground ! |
| ! ! Comm-B ! station ! |
| ! MV ! MESSAGE ! Contains ARA, RAC, VDS subfields used for ! |
| ! ! Comm-V ! coordination ! |
| ! RI ! REPLY ! Type of reply and airspeed capability ! |
| ! ! INFORMATION ! ! |
| ! SL ! SENSITIVITY ! TCAS current sensitivity level ! |
| ! ! LEVEL ! ! |
| ! UM ! UTILITY MESSAGE! Transponder status readouts ! |
| ! VS ! VERTICAL STATUS! Aircraft status: 0 = airborne, 1 = ground ! |
| ------------------------------------------------------------------------------- |
B. Measurement of Intruder Parameters
(1) Principle
(a) Determination of relative altitude
Upon confirmed transponder reception, the TCAS starts to interrogate the intruder. Its altitude is transmitted directly in the reply (standard barometric altitude) and this information is used to determine the relative altitude of the two aircraft, by calculating the barometric altitude difference.
This computation is, however, only possible with respect to Mode C or Mode S transponder-equipped aircraft.
Upon confirmed transponder reception, the TCAS starts to interrogate the intruder. Its altitude is transmitted directly in the reply (standard barometric altitude) and this information is used to determine the relative altitude of the two aircraft, by calculating the barometric altitude difference.
This computation is, however, only possible with respect to Mode C or Mode S transponder-equipped aircraft.
(b) Range measurement
The range is calculated by measuring the elapsed time between transmission of the interrogation signal and return of the reply transmitted by the intruder. Aircraft are detected from a minimum range of 14 NM.
The range is calculated by measuring the elapsed time between transmission of the interrogation signal and return of the reply transmitted by the intruder. Aircraft are detected from a minimum range of 14 NM.
(c) Determination of Azimuth
There are several methods for calculating the angle of reception of a radio-electric signal with respect to a reference direction such as the aircraft centerline.
The TCAS utilizes a four-element antenna and a form of phase interferometry to derive bearing from the received signal.
The interferometry principle is based on a comparison of signal phases received by four independent elements of the directional antenna.
For any given direction of reception, three antenna elements and their corresponding receiver channels are used. The elements which are used are the one physically positioned in the center of the quadrant being interrogated and the elements adjacent to it.
Two phase measurements are performed: one corresponding to the difference in phase between the center element and the element on its right, and one corresponding to the phase difference between the center element and the element on its left. These two signals are converted from analog to digital information and stored. The bearing is calculated by reading the phase detector outputs, taking the ratio of the phase detector outputs with respect to each other, and performing a table lookup which relates the ratio to the phase difference from beam center.
There are several methods for calculating the angle of reception of a radio-electric signal with respect to a reference direction such as the aircraft centerline.
The TCAS utilizes a four-element antenna and a form of phase interferometry to derive bearing from the received signal.
The interferometry principle is based on a comparison of signal phases received by four independent elements of the directional antenna.
For any given direction of reception, three antenna elements and their corresponding receiver channels are used. The elements which are used are the one physically positioned in the center of the quadrant being interrogated and the elements adjacent to it.
Two phase measurements are performed: one corresponding to the difference in phase between the center element and the element on its right, and one corresponding to the phase difference between the center element and the element on its left. These two signals are converted from analog to digital information and stored. The bearing is calculated by reading the phase detector outputs, taking the ratio of the phase detector outputs with respect to each other, and performing a table lookup which relates the ratio to the phase difference from beam center.
(d) Tracking
Once identified, the intruders are tracked by a series of interrogation-replies in Mode C only all-call for Mode C transponder-equipped aircraft, and in Mode S for Mode S transponder-equipped aircraft.
These exchanges permit the TCAS to periodically update the altitude, range and bearing data for each intruder and to compute the range rate and altitude rate variations. These data are then used to determine the time separating the two aircraft from their closest point of approach.
Once identified, the intruders are tracked by a series of interrogation-replies in Mode C only all-call for Mode C transponder-equipped aircraft, and in Mode S for Mode S transponder-equipped aircraft.
These exchanges permit the TCAS to periodically update the altitude, range and bearing data for each intruder and to compute the range rate and altitude rate variations. These data are then used to determine the time separating the two aircraft from their closest point of approach.
(e) Broadcast messages
Every ten seconds, the TCAS transmits a broadcast message intended to inform nearby aircraft, themselves equipped with a TCAS, of the presence of a TCAS-equipped aircraft in their traffic area.
These messages, received by the Mode S transponders, are communicated to the TCAS computer to enable it to know the number of TCAS-equipped aircraft in its detection envelope. This information is then used in the interference limitation formulas whose results modulate the Mode S interrogation output power level in inverse proportion to the number of aircraft. This reduces the number of non-elicited replies received by ground ATC stations.
The messages transmitted are of the uplink format type UF = 16 with the Mode S 24 bit address of the interrogating TCAS included in the MID field (bits 65 to 88) with the UDS field (bits 33 to 40) containing the code F50. No response is expected for this type of message.
Every ten seconds, the TCAS transmits a broadcast message intended to inform nearby aircraft, themselves equipped with a TCAS, of the presence of a TCAS-equipped aircraft in their traffic area.
These messages, received by the Mode S transponders, are communicated to the TCAS computer to enable it to know the number of TCAS-equipped aircraft in its detection envelope. This information is then used in the interference limitation formulas whose results modulate the Mode S interrogation output power level in inverse proportion to the number of aircraft. This reduces the number of non-elicited replies received by ground ATC stations.
The messages transmitted are of the uplink format type UF = 16 with the Mode S 24 bit address of the interrogating TCAS included in the MID field (bits 65 to 88) with the UDS field (bits 33 to 40) containing the code F50. No response is expected for this type of message.
(f) Communications frequencies
Communications between two aircraft are always crossed between transponder and TCAS. The TCAS transmits at a frequency of 1030 MHz to the transponder of the other aircraft, whose reply signals are at a frequency of 1090 MHz to the TCAS receiver. This choice allows system compatibility with ground station-transponder links as the ground stations use the same frequencies as the TCAS.
Communications between two aircraft are always crossed between transponder and TCAS. The TCAS transmits at a frequency of 1030 MHz to the transponder of the other aircraft, whose reply signals are at a frequency of 1090 MHz to the TCAS receiver. This choice allows system compatibility with ground station-transponder links as the ground stations use the same frequencies as the TCAS.
C. Coordination
Two TCAS-equipped aircraft must coordinate their maneuvers to avoid the flight path corrections ordered by each TCAS resulting in a hazardous situation.
Two TCAS-equipped aircraft must coordinate their maneuvers to avoid the flight path corrections ordered by each TCAS resulting in a hazardous situation.
(1) Coordination principle
In most cases of encounters between two TCAS-equipped aircraft, mutual identification is almost but not quite simultaneous, with sufficient time lag to establish the priority necessary for the coordination process.
The first aircraft to detect a potentially dangerous configuration computes a deviation maneuver sense and communicates it to the other aircraft.
This aircraft takes the information into account and in turn computes a correction.
If two aircraft detect each other at exactly the same time and simultaneously transmit coordination messages containing incompatible deviation senses it is, by convention, the aircraft that has the highest Mode S address that cancels its trajectory correction. A time delay in the display of orders on the display units avoids opposing orders.
In most cases of encounters between two TCAS-equipped aircraft, mutual identification is almost but not quite simultaneous, with sufficient time lag to establish the priority necessary for the coordination process.
The first aircraft to detect a potentially dangerous configuration computes a deviation maneuver sense and communicates it to the other aircraft.
This aircraft takes the information into account and in turn computes a correction.
If two aircraft detect each other at exactly the same time and simultaneously transmit coordination messages containing incompatible deviation senses it is, by convention, the aircraft that has the highest Mode S address that cancels its trajectory correction. A time delay in the display of orders on the display units avoids opposing orders.
(2) Communications protocol
Communications between two aircraft comprises three phases:
Communications between two aircraft comprises three phases:
(a) Detection phase
The TCAS receives squitter messages transmitted by the transponder of the intruder aircraft. These are DF11-type messages transmitted periodically at one second intervals and intended to enable its detection and identification.
This message essentially uses two fields:
The TCAS receives squitter messages transmitted by the transponder of the intruder aircraft. These are DF11-type messages transmitted periodically at one second intervals and intended to enable its detection and identification.
This message essentially uses two fields:
- an AA (Address Announced) field containing the Mode S address which must be acknowledged
- A PI (parity interrogator Identity) field which is the result of a parity encoding using a method of polynominal multiplication. On reception, the Address Announced field is multiplied by the same polynominal and the result is compared with the received value.
(b) Surveillance phase
After detection, a surveillance phase starts. This phase is composed of an acquisition part and a tracking part.
After detection, a surveillance phase starts. This phase is composed of an acquisition part and a tracking part.
1 Acquisition interrogation
When the TCAS receives a squitter and acquires the Mode S address of the intruder it enters into contact by transmitting a UFO-type message (Short special surveillance interrogation) with the following specific fields:
When the TCAS receives a squitter and acquires the Mode S address of the intruder it enters into contact by transmitting a UFO-type message (Short special surveillance interrogation) with the following specific fields:
- bit 9, RL = 0: reply message length requested short (56 bits)
- bit 14, AQ = 1: acquisition-type message indication.
2 Acquisition reply
The intruder's transponder replies to this request by a DFO message containing the following information:
The intruder's transponder replies to this request by a DFO message containing the following information:
- bit 6, VS: Vertical Status, = 1 if the aircraft is on the ground, = 0 if the aircraft is airborne
- bits 9 to 11, SL: Sensitivity level indicates in which sensitivity level its TCAS is operating
- bits 14 to 17, RI: combinations of bits, from values 8 to 15, specify the maximum speed the aircraft can reach. The other combinations are not used
- bits 20 to 32, AC: aircraft altitude code indicating the barometric altitude.
3 Tracking interrogation
After its acquisition, the intruder is tracked by UFO-type interrogations with the following field values:
After its acquisition, the intruder is tracked by UFO-type interrogations with the following field values:
- RL = 0: reply message length requested short
- AQ = 0: not an acquisition message.
4 Tracking reply
The intruder's transponder replies with a DFO message indicating altitude and TCAS sensitivity level by a combination of fields SL and RI:
The intruder's transponder replies with a DFO message indicating altitude and TCAS sensitivity level by a combination of fields SL and RI:
- SL: bits 9 to 11
- RI: bits 14 to 17, combination values 0 to 7. Values 8 to 15 are not used
- AC: bits 20 to 32, aircraft altitude code.
(c) Coordination phase
If the intruder becomes a threat, the TCAS programs a deviation maneuver to avoid a risk of collision. A coordination procedure is initiated between the two aircraft with an exchange of the following messages:
If the intruder becomes a threat, the TCAS programs a deviation maneuver to avoid a risk of collision. A coordination procedure is initiated between the two aircraft with an exchange of the following messages:
1 Coordination interrogation
The TCAS transmits a UF16 Long Special Surveillance message whose fields contain the following indications:
The TCAS transmits a UF16 Long Special Surveillance message whose fields contain the following indications:
| --------------------------------------------------------------------- |
| ! BITS ! FIELD ! INDICATION ! |
| !-------!--------!--------------------------------------------------! |
| ! 9 ! RL ! = 1: reply message length requested long ! |
| ! 14 ! AQ ! = 0: non-acquisition type interrogation ! |
| ! 33-40 ! UDS ! U Definition Subfield - defines the other data ! |
| ! ! ! in the MU field (Comm-U), composed of bits 42 ! |
| ! ! ! to 88 ! |
| ! 42 ! MTB ! Indicates multiple threat processing ! |
| ! 43-44 ! CVC ! Cancel Vertical resolution advisory Complement - ! |
| ! ! ! used to cancel an RA complement sent earlier to ! |
| ! ! ! an intruder ! |
| ! 45-46 ! VRC ! Vertical Resolution advisory Complement - used to! |
| ! ! ! transmit an RA vertical complement to the ! |
| ! ! ! intruder requesting it not to modify its ! |
| ! ! ! trajectory (don't climb, don't descend) ! |
| ! 47-49 ! CHC ! Cancel Horizontal resolution advisory Complement-! |
| ! ! ! not used in TCAS II ! |
| ! 50-52 ! HRC ! Horizontal Resolution advisory Complement - not ! |
| ! ! ! used in TCAS II ! |
| ! 53-55 ! ! not used ! |
| ! 56-60 ! HSB ! Encoded Sense bits for Horizontal resolution ! |
| ! ! ! advisory complement - not used in TCAS II ! |
| ! 61-64 ! VSB ! Encoded Sense Bits for Vertical resolution advi- ! |
| ! ! ! sory complement - parity code to protect the 4 ! |
| ! ! ! vertical command bits (43-46) ! |
| ! 65-88 ! MID ! Interrogator TCAS-equipped aircraft Mode S ! |
| ! ! ! address. ! |
| --------------------------------------------------------------------- |
2 Coordination reply
After acquisition of this message, the intruder's transponder replies with a Long Special Surveillance DF 16 type message, containing the information previously transmitted to it by its own TCAS:
After acquisition of this message, the intruder's transponder replies with a Long Special Surveillance DF 16 type message, containing the information previously transmitted to it by its own TCAS:
| --------------------------------------------------------------------- |
| ! BITS ! FIELD ! INDICATION ! |
| !------ !-------!---------------------------------------------------! |
| ! 6 ! VS ! Vertical Status - indicates whether aircraft is ! |
| ! ! ! on ground or airborne ! |
| ! 9-11 ! SL ! With RI, SL indicates the sensitivity level at ! |
| ! ! ! which the interrogated aircraft's TCAS is ! |
| ! ! ! operating ! |
| ! 14-17 ! RI ! Reply Information ! |
| ! 20-32 ! AC ! Altitude Code - contains aircraft altitude ! |
| ! ! ! encoded in 100 ft increments if bit 28 equals 0, ! |
| ! ! ! and in 25 ft increments if bit 28 equals 1 ! |
| ! 33-40 ! VDS ! V Definition Subfield defines the contents of the! |
| ! ! ! data and coding in the field MV (Comm V) composed! |
| ! ! ! of bits 41 to 88 ! |
| ! 41-54 ! ARA ! Active Resolution Advisory - indicates the RA ! |
| ! ! ! type currently generated by the TCAS ! |
| ! 55-58 ! RAC ! Resolution Advisory Complement - Indicates the RA! |
| ! ! ! complement type currently received from other ! |
| ! ! ! TCAS-equipped aircraft ! |
| ! 59-88 ! ! Not used ! |
| --------------------------------------------------------------------- |
(d) Correspondence of radio messages with ARINC 429 words
The coordination information exchanged between two aircraft transits via ARINC 429 buses linking the transponder to the TCAS. The following table gives the correspondence between the two types of link:
The coordination information exchanged between two aircraft transits via ARINC 429 buses linking the transponder to the TCAS. The following table gives the correspondence between the two types of link:
| --------------------------------------------------- |
| ! 1 - TRANSPONDER TO TCAS ! |
| !-------------------------------------------------! |
| ! ! RADIO MESSAGE ! ARINC MESSAGE ! |
| !---------!-------------------!-------------------! |
| ! FIELD ! MESSAGE ! BITS ! LABEL ! BITS ! |
| !---------!---------!---------!---------!---------! |
| ! MTB ! UF16 ! 42 ! 271 ! 09 ! |
| ! CVC ! ! 43-44 ! ! 10-11 ! |
| ! VRC ! ! 45-46 ! ! 12-13 ! |
| ! CHC ! ! 47-49 ! ! 14-16 ! |
| ! HRC ! ! 50-52 ! ! 17-19 ! |
| ! HSB ! ! 56-60 ! ! 20-24 ! |
| ! VSB ! ! 61-64 ! ! 25-28 ! |
| --------------------------------------------------- |
| --------------------------------------------------- |
| ! 2 - TCAS TO TRANSPONDER ! |
| !-------------------------------------------------! |
| ! ! RADIO MESSAGE ! ARINC MESSAGE ! |
| !---------!-------------------!-------------------! |
| ! FIELD ! MESSAGE ! BITS ! LABEL ! BITS ! |
| !---------!---------!---------!---------!---------! |
| ! SL ! DF16 ! 09-11 ! 274 ! 23-25 ! |
| ! RI ! ! 14-17 ! ! 26-29 ! |
| ! ARA ! ! 41-54 ! 273 ! 12-25 ! |
| ! RAC ! ! 55-58 ! ! 26-29 ! |
| --------------------------------------------------- |
D. Principles of Computation
In the TCAS, target aircraft are categorized depending on specific criteria varying in function of altitude.
The TCAS essentially uses two types of information to perform this classification:
In the TCAS, target aircraft are categorized depending on specific criteria varying in function of altitude.
The TCAS essentially uses two types of information to perform this classification:
- the relative altitude between two aircraft, known by the difference of their barometric altitudes
- the distance or range separating them.
Acquisition of these two parameters at regular intervals (tracking) enables their variations to be calculated: - altitude rate
- range rate.
Assessment of the potential threat represented by an intruder depends on two criteria determined with respect to a point in the traffic area called Closest Point of Approach (CPA).
This is the point of minimum distance between the two aircraft, assuming that their trajectories do not deviate.
The two criteria are: - vertical separation at CPA
- time left before reaching CPA.
(1) Projected vertical separation
The threat is evaluated by calculating the vertical separation between the two aircraft at the closest point of approach.
The TCAS computer processes the current altitude and altitude rate of the intruder to predict whether it will be within limits considered dangerous at the closest point of approach.
Intruder 1 penetrates the altitude zone delimited by the upper and lower thresholds but when it reaches CPA, it will be outside this zone.
Therefore no advisory will be issued.
However, when intruder 2 reaches CPA it is still inside this zone and therefore an advisory will be issued.
The threat is evaluated by calculating the vertical separation between the two aircraft at the closest point of approach.
The TCAS computer processes the current altitude and altitude rate of the intruder to predict whether it will be within limits considered dangerous at the closest point of approach.
Intruder 1 penetrates the altitude zone delimited by the upper and lower thresholds but when it reaches CPA, it will be outside this zone.
Therefore no advisory will be issued.
However, when intruder 2 reaches CPA it is still inside this zone and therefore an advisory will be issued.
(2) Time to intercept (TAU)
The TCAS does not need to locate the CPA in space, but rather it needs to know the time to intercept for two aircraft. For example, if two aircraft are approaching on the same axis on a collision course, this time is the ratio of distance between them to the sum of their speeds.
With the risk of collision being in inverse proportion to this time, trajectory correction orders are initiated by crossing predetermined time thresholds whose values depend on the altitude layer in which the aircraft is located.
This method of calculation avoids the initiation of corrections if, from a certain distance, the TAU trend is inverted even though the distance separating the two aircraft decreases. For example, in the case of two aircraft moving on parallel axes but in the opposite direction.
The curve represents the TAU variation before the closest point of approach.
This time decreases as the range decreases, reaches a minimum when the intruder's relative bearing is 45 deg. and then increases rapidly. If the time corresponding to a predetermined threshold has not been reached before the minimum TAU value, a trajectory correction will not be initiated.
Certain conditions exist, however, in which this formula may prove insufficient. This may arise for example when two aircraft fly in the same direction with a very low closure rate. In this case, the range/range rate ratio gives a high TAU value whereas in fact, the two aircraft could be very near.
The TCAS uses a slightly different formula to preclude this situation:
This formula represents an estimate of the time the aircraft would take to penetrate a sphere around the intruder with a radius equal to (DMOD)expnt2/range. The DMOD coefficients are in function of altitude, ranging from 0.20 NM for 1000-2350ft, to 1.1 NM above 20,000 ft as shown below:
The TCAS does not need to locate the CPA in space, but rather it needs to know the time to intercept for two aircraft. For example, if two aircraft are approaching on the same axis on a collision course, this time is the ratio of distance between them to the sum of their speeds.
| D |
| T = --------- |
| V1 + V2 |
| More generally, the TCAS uses range and range rate measurement to |
| compute this time: |
| RANGE R(NM) |
| TAU = ------------ i.e. TAU(s) = 3600 -------- |
| RANGE RATE RR (Kts) |
This method of calculation avoids the initiation of corrections if, from a certain distance, the TAU trend is inverted even though the distance separating the two aircraft decreases. For example, in the case of two aircraft moving on parallel axes but in the opposite direction.
The curve represents the TAU variation before the closest point of approach.
This time decreases as the range decreases, reaches a minimum when the intruder's relative bearing is 45 deg. and then increases rapidly. If the time corresponding to a predetermined threshold has not been reached before the minimum TAU value, a trajectory correction will not be initiated.
Certain conditions exist, however, in which this formula may prove insufficient. This may arise for example when two aircraft fly in the same direction with a very low closure rate. In this case, the range/range rate ratio gives a high TAU value whereas in fact, the two aircraft could be very near.
The TCAS uses a slightly different formula to preclude this situation:
| range - X (DMOD) to the power two |
| TAU = ---------- with X = --------- |
| range rate range |
| Altitude DMOD |
| (ft) (NM) |
| 0-1000 |
| 1000-2350 0.20 |
| 2350-5000 0.35 |
| 5000-10000 0.55 |
| 10000-20000 0.80 |
| 20000-42000 1.10 |
| above 42000 1.10 |
E. Definition of Target Aircraft
Target aircraft are divided into four categories: OTHER - PROXIMATE - TRAFFIC ADVISORY (TA) - RESOLUTION ADVISORY (RA).
Target aircraft are divided into four categories: OTHER - PROXIMATE - TRAFFIC ADVISORY (TA) - RESOLUTION ADVISORY (RA).
(1) Other aircraft
Depending on the Above/Below selection, targets are defined as other intruders if their:
Depending on the Above/Below selection, targets are defined as other intruders if their:
- altitude is between 2700 ft and 9900 ft above or below the TCAS equipped aircraft
- distance is more than 14 NM.
Their presentation on the ND is permanent (TFC function selected = full time display function activated) or conditioned by the presence of a TA/RA intruder (TA or TA/RA function selected).
Depending on their trajectory, they may: - conserve this status and move away without an advisory being declared.
In this case the pilot is informed of their presence on the ND by a
white outlined diamond symbol and can monitor their progress, or - have a trajectory liable to lead to a conflict situation and in this case they require a new category and a symbol change.
(2) Proximate aircraft
Targets are defined as proximate traffic if the difference between their altitude and that of the TCAS aircraft is less than 1200 ft and if their range is within 6 NM.
Their presentation on the ND is permanent (TFC function selected = full time display function activated) or conditioned by the presence of a TA/RA intruder (TA or TA/RA function selected).
Generally aircraft not in the immediate vicinity enter into this category.
Depending on their trajectory, they may:
Targets are defined as proximate traffic if the difference between their altitude and that of the TCAS aircraft is less than 1200 ft and if their range is within 6 NM.
Their presentation on the ND is permanent (TFC function selected = full time display function activated) or conditioned by the presence of a TA/RA intruder (TA or TA/RA function selected).
Generally aircraft not in the immediate vicinity enter into this category.
Depending on their trajectory, they may:
- conserve this status and move away without an advisory being declared.
In this case the pilot is informed of their presence on the ND by a white filled diamond symbol and can monitor their progress, or - have a trajectory liable to lead to a conflict situation and in this case they require a traffic advisory and their symbol changes.
(3) Traffic advisory aircraft
When an intruder is relatively near but does not represent an immediate threat the TCAS issues a traffic advisory. Its presence is displayed on the ND by an amber filled circle. Its display is accompanied by an aural alert:
"Traffic Traffic".
The pilot is therefore aware of its presence and knows its range and relative bearing. Its display is linked to vertical separation and time TAU before CPA values.
Depending on its trajectory, an intruder may conserve this status and move away, or it may become a collision threat. In this case avoidance maneuvers are suggested to the pilot via a resolution advisory.
When an intruder is relatively near but does not represent an immediate threat the TCAS issues a traffic advisory. Its presence is displayed on the ND by an amber filled circle. Its display is accompanied by an aural alert:
"Traffic Traffic".
The pilot is therefore aware of its presence and knows its range and relative bearing. Its display is linked to vertical separation and time TAU before CPA values.
Depending on its trajectory, an intruder may conserve this status and move away, or it may become a collision threat. In this case avoidance maneuvers are suggested to the pilot via a resolution advisory.
(4) Resolution advisory aircraft
In resolution displays, the intruder is represented on the ND by a red filled square and corrective orders are issued on the vertical speed scale of the PFD. Crossing into resolution advisory occurs for a TAU time threshold 10 to 15 seconds lower than a traffic advisory threshold.
Vertical separation between the two aircraft is also taken into account for this category. There are two types of resolution advisory, in function of the vertical separation value:
In resolution displays, the intruder is represented on the ND by a red filled square and corrective orders are issued on the vertical speed scale of the PFD. Crossing into resolution advisory occurs for a TAU time threshold 10 to 15 seconds lower than a traffic advisory threshold.
Vertical separation between the two aircraft is also taken into account for this category. There are two types of resolution advisory, in function of the vertical separation value:
- Preventive Advisories
- Corrective Advisories.
(a) Preventive Advisory
In this case the vertical separation is less than a threshold S1 and greater than a threshold S2. The advisory instructs the pilot to avoid certain deviations from current vertical rate, this measure being sufficient to avoid a risk of collision. On the PFD speed scale, the forbidden values are indicated by red sectors.
In this case the vertical separation is less than a threshold S1 and greater than a threshold S2. The advisory instructs the pilot to avoid certain deviations from current vertical rate, this measure being sufficient to avoid a risk of collision. On the PFD speed scale, the forbidden values are indicated by red sectors.
(b) Corrective advisory
In this case, the vertical separation is lower than the threshold S2.
On the vertical speed scale of the PFD, colored sectors depict avoidance maneuvers to be performed:
In this case, the vertical separation is lower than the threshold S2.
On the vertical speed scale of the PFD, colored sectors depict avoidance maneuvers to be performed:
- red sector = forbidden vertical speeds
- green "fly to" sector = a vertical speed range to be respected.
(5) Aural alerts
Trajectory correction or holding visual orders are accompanied by synthesized voice announcements whose level cannot be adjusted by the pilot. These announcements are generated by the TCAS computer and broadcast via the cockpit loud speakers. These messages and their meanings are described below:
Trajectory correction or holding visual orders are accompanied by synthesized voice announcements whose level cannot be adjusted by the pilot. These announcements are generated by the TCAS computer and broadcast via the cockpit loud speakers. These messages and their meanings are described below:
- "CLIMB, CLIMB":
Climb at the rate shown by the green sector on the PFD (1500 ft/min), - "CLIMB, CROSSING CLIMB, CLIMB, CROSSING CLIMB":
As above except that it further indicates that own flight path will cross through that of the intruder, - "INCREASE CLIMB, INCREASE CLIMB":
Follows a "climb" advisory. The vertical speed of the aircraft should be increased (2500 ft/min), - "CLIMB, CLIMB NOW, CLIMB, CLIMB NOW":
Follows a "descend" advisory when a reversal in sense is required to achieve safe vertical separation from a maneuvering intruder, - "DESCEND, DESCEND":
Descend at the rate indicated by the green sector on the PFD (-1500 ft/min), - "DESCEND, CROSSING DESCEND, DESCEND, CROSSING DESCEND":
As above except that it further indicates that own flight path will cross through that of the intruder, - "INCREASE DESCENT, INCREASE DESCENT":
Follows a "descend" advisory. The vertical speed of the descent should be increased (-2500 ft/min), - "DESCEND, DESCEND NOW, DESCEND, DESCEND NOW":
Follows a "climb" advisory when a reversal in sense is required to achieve safe vertical separation from a maneuvering intruder, - "LEVEL-OFF, LEVEL-OFF":
Reduce vertical speed to level off as shown by the green sector on the PFD.
It can correspond to a corrective reduce climb or reduce descent.
It can also represent a weakening of corrective RA.
- "MONITOR VERTICAL SPEED":
Indicates that a forbidden vertical speed range exists (red sector) and that pilot must monitor vertical speed so as not to enter this range (Preventive Advisory). - "MAINTAIN VERTICAL SPEED, MAINTAIN":
Indicates a non-crossing advisory type, maintains rate RA's (corrective). - "MAINTAIN VERTICAL SPEED, CROSSING MAINTAIN":
Indicates an altitude crossing advisory type, maintains rate RA's (corrective).
These messages are spoken only once if softening from a previous corrective advisory, - "CLEAR OF CONFLICT":
Indicates that separation has been achieved and range has started to increase.
(6) Advisory inhibit conditions
In certain particular conditions, certain advisories are not generated as they could lead to the pilot adopting flight conditions that are hazardous or outside the aircraft's performance capability.
In certain particular conditions, certain advisories are not generated as they could lead to the pilot adopting flight conditions that are hazardous or outside the aircraft's performance capability.
(a) Low altitude inhibitions
Ground proximity leads to the inhibition of those advisories liable to cause a hazardous situation at this level. In decreasing altitude, these are:
Ground proximity leads to the inhibition of those advisories liable to cause a hazardous situation at this level. In decreasing altitude, these are:
- below 1550 +/- 100 ft above ground level (AGL) inhibition of "Increase Descend" resolution advisories (RA),
- below 1100 +/- 100 ft AGL, inhibition of "Descend" resolution advisories (RA),
- below 1000 ft +/- 100 ft AGL, TCAS automatic switching to TA Only mode (inhibition of all resolution advisories),
- below 500 +/- 100 ft AGL (600 ft in climb, 400 ft in descent), inhibition of all resolution advisories (RA) and of aural traffic advisories (TA).
(b) High altitude inhibition
Above 48,000 ft, further climb orders are inapplicable as the aircraft performance capability does not permit them to be taken into account.
"Climb" advisories are therefore inhibited above this altitude.
Above 48,000 ft, further climb orders are inapplicable as the aircraft performance capability does not permit them to be taken into account.
"Climb" advisories are therefore inhibited above this altitude.
(c) Advisory inhibit discretes
Three discretes are used to manage priority between:
Three discretes are used to manage priority between:
- windshear/stall,
- GPWS - G/S,
- and the TCAS computer.
The environmental alert priorities are: windshear/stall, GPWS - G/S and then TCAS II.
When TCAS II is inhibited, the TA ONLY mode is selected and the voice announcements are cancelled.
(d) Rejection of signals from aircraft on ground
Intruders declared to be on-ground can never cause proximate, traffic or resolution advisory (TA, RA.) and are not displayed on the Navigation Display.
Determination of Airborne/On ground status:
In the figure, the intruder's height above ground level can be deduced:
Intruders declared to be on-ground can never cause proximate, traffic or resolution advisory (TA, RA.) and are not displayed on the Navigation Display.
Determination of Airborne/On ground status:
- If the intruder is equipped with a Mode S transponder, the ground/flight status is sent in the DF message. The TCAS 7.0 directly knows if this intruder is on the ground.
NOTE: Former version of the TCAS applied the conversion algorithm as described below.
- If the intruder's ground/flight status is not directly available (Mode C Transponder for instance), the TCAS shall process the barometric altitude broadcast by the intruder (process described here below) to convert it into height above ground level. Then, the intruder is declared to be on-ground if it is below 360 ft AGL when descending, and 400 ft AGL when climbing (380 +/- 20 ft).
In the figure, the intruder's height above ground level can be deduced:
| Z i/s = Z t/s -(Z t/m - Z i/m) |
| where: Z i/s = height of intruder above ground level |
| Z i/m = altitude of intruder above sea level |
| Z t/s = height of TCAS above ground level measured |
| by radio altimeter |
| Z t/m = altitude of TCAS above sea level measured |
| by ADIRU. |
F. Sensitivity Level
The notion of sensitivity level is very important in the TCAS as many of the operating modes depend on it.
The TCAS separates the surrounding airspace into altitude layers. A different Sensitivity Level (SL) threshold for issuing advisories is applied to each altitude layer.
The sensitivity level is decreased at low altitude to prevent unnecessary advisories in higher traffic densities such as terminal areas.
Generally, the level is determined automatically by the TCAS in function of:
The notion of sensitivity level is very important in the TCAS as many of the operating modes depend on it.
The TCAS separates the surrounding airspace into altitude layers. A different Sensitivity Level (SL) threshold for issuing advisories is applied to each altitude layer.
The sensitivity level is decreased at low altitude to prevent unnecessary advisories in higher traffic densities such as terminal areas.
Generally, the level is determined automatically by the TCAS in function of:
- altitude values from the radio altimeter up to 2500 ft AGL,
- barometric altitude values in the 2500 ft to 48,000 ft range. TAU values corresponding to each sensitivity level indicate the TA and RA thresholds. The vertical separation thresholds at CPA also vary in function of the sensitivity level for the different types of advisory.
The following table summarizes these data:---------------------------------------------------------------------- ! ! TAU thresholds ! Vertical separation ! ! ! ! thresholds ! !----------------------------!-----------------!---------------------! ! Source ! Altitude ! SL TA RA ! S0 S1 S2 ! ! Altitude ! ! ! TA RA RA ! ! ! ! ! prev cor ! ! ! ! (sec) (sec) ! (ft) (ft) (ft) ! !------------!---------------!-----------------!---------------------! ! Radio Alt !less than 1000 ! 2 20 ! 850 ! ! Radio Alt ! 1000-2350 ! 3 25 15 ! 850 600 300 ! ! Baro ! 2350-5000 ! 4 30 20 ! 850 600 300 ! ! Baro ! 5000-10000 ! 5 40 25 ! 850 600 350 ! ! Baro ! 10000-20000 ! 6 45 30 ! 850 600 400 ! ! Baro ! 20000-42000 ! 7 48 35 ! 850 700 600 ! ! Baro !more than 42000! 7 48 35 ! 1200 800 700 ! ----------------------------------------------------------------------
NOTE: Level 1 corresponds to Standby Mode in which no advisory is generated.
NOTE: This table indicates the threshold based on own aircraft altitude. Each aircraft altitude depends on an hysteresis:
- 1000 plus or minus 100 ft,
- 2350 plus or minus 200 ft,
- 5000 plus or minus 500 ft,
- 10,000 plus or minus 500 ft,
- 20,000 plus or minus 500 ft,
- 42,000 plus or minus 500 ft.
For example, to switch from sensitivity level 3 to sensitivity level 2, the altitude must fall below 900 ft.
However, to switch from sensitivity level 2 to sensitivity level 3, the altitude value must go above 1100 ft.
- selecting TA only mode on the ATC/TCAS control unit forces level 2. In this case, intruders of all types are displayed but will not be transformed into RA symbols and no vertical speed modification indications will be issued.
- the ATC/Mode S equipped ground stations may modify the sensitivity level of the aircraft TCAS via the uplink without, however, having the capability to force the Standby Mode. If several ground stations command sensitivity levels, the TCAS logic selects the lowest level.
Definition of priority logic:
First a sensitivity level based on altitude is selected. Level 2 is selected if the radio altimeter altitude is less than 1000 ft. Level 2 is also selected if own aircraft is configured such that both CLIMB and DESCEND RAs are inhibited. Level 3 is selected if the aircraft is above 1000 ft and below 2350 ft AGL. If the aircraft is above 2350 ft AGL, barometric altitude is used to select either level 4 (below 5000 ft), 5 (from 5000 to 10,000 ft), 6 (from 10,000 to 20,000 ft), and 7 (above 20,000 ft).
ATC/TCAS control unit input is read by the TCAS computer. If the pilot has selected Automatic Mode (TA/RA), then the altitude-based sensitivity level will be used in comparisons to determine the final level.
From all sensitivity level commands, if any, received from ground stations, the lowest is selected.
If the TA ONLY mode is selected, either manually via the control unit or by a ground station, the altitude-based sensitivity level is used for TA thresholds and the RAs are inhibited. Otherwise, the lowest of all inputs is chosen.
G. Information Display
The TCAS information is presented on the CAPT and F/O NDs of the EFIS system. Additional messages can also be presented on the display units of the ECAM system.
The TCAS information is presented on the CAPT and F/O NDs of the EFIS system. Additional messages can also be presented on the display units of the ECAM system.
(1) Traffic information on the ND
Target aircraft are presented on the ND in ROSE or ARC mode but not in PLAN mode.
These traffic indications show the situation in the surveillance zone.
The aircraft present in this zone are represented by symbols whose shape and color correspond to the type of intruders defined in the TCAS.
The symbols are positioned on the ND so as to depict their relative bearing and range. Data tags are associated with intruders.
These tags consist of:
Target aircraft are presented on the ND in ROSE or ARC mode but not in PLAN mode.
These traffic indications show the situation in the surveillance zone.
The aircraft present in this zone are represented by symbols whose shape and color correspond to the type of intruders defined in the TCAS.
The symbols are positioned on the ND so as to depict their relative bearing and range. Data tags are associated with intruders.
These tags consist of:
- two digits indicating their relative altitude in hundreds of feet
- a symbol indicating whether the intruder is above (+) or below (-) the aircraft.
An arrow to the right of the symbol indicates the vertical trend of the aircraft.
Targets are symbolized according to their type: - OTHER TRAFFIC: white outlined diamond, height 7 mm
- PROXIMATE TRAFFIC: white filled diamond, height 7 mm
- TRAFFIC ADVISORY: amber filled circle, diameter 5 mm
- RESOLUTION ADVISORY: red filled square, side 5 mm.
The display only presents the eight most threatening intruders (number determined through program pins on the TCAS computer).
The own aircraft is represented by the aircraft symbol at the center of the dial in ROSE mode and at the lower quarter and at the center of the image in ARC mode.
A white range ring with markings at each of the twelve clock positions is placed around the own aircraft symbol at a radius of 2.5 NM.
The following figure shows an example of the display on the ND:
These indications are only presented for the 10, 20 and 40 NM range selections. If a TA or RA type intruder is detected and the display range is at a higher scale, the following message comes into view at the center of the display, in red for RA and in amber for TA:
REDUCE RANGE
If a TA or RA type intruder is detected and the ND mode is inadequate for display, the following message comes into view at the center of the display in the same colors as above:
CHANGE MODE
The TCAS can detect an intruder without acquiring its bearing (for instance, multipath problem). In this case its range, relative altitude and an arrow are displayed in the TCAS area (at the bottom of the ND).
The color of the display is the same as the color of the intruder symbol.
OFF SCALE intruder: if only a half of a symbol is displayed, the target is defined as an off scale intruder. This calls for increasing the range selection on the ND.
(2) Aircraft control information on the PFD
Resolution advisories are represented on the vertical speed scale of the PFD by indications given in the form of a band made up of colored sectors:
Resolution advisories are represented on the vertical speed scale of the PFD by indications given in the form of a band made up of colored sectors:
- a red sector represents a forbidden vertical speed range
- a green sector indicates the vertical speed range the aircraft should fly in to avoid a collision threat represented by one or more intruders.
(3) Corrective advisory display
Corrective resolution advisories are displayed to advise the crew to perform an avoidance maneuver in the vertical sense.
This maneuver may take different forms:
Corrective resolution advisories are displayed to advise the crew to perform an avoidance maneuver in the vertical sense.
This maneuver may take different forms:
- climb or descent if the aircraft is in level flight
- reducing or increasing rate of climb or reversing to descent if the aircraft is in climb
- reducing or increasing rate of descent or reversing to climb if the aircraft is in descent.
When resolution advisories are displayed, the vertical speed scale surface changes from trapezoidal to rectangular.
The grey background is replaced by green and red sectors defining the optimum vertical speed values.
The pilot's task is to maneuver the aircraft to keep the needle out of the red sectors and place it in the adjacent green "Fly-to" sector.
The vertical speed information needle and digits are colored in red when the vertical speed is in the forbidden area. It becomes green when the vertical speed is in the authorized area.
(4) Preventive advisory display
Preventive resolution advisories advise the pilot to avoid vertical speeds that could lead to a hazardous situation.
They are represented by one or two red sectors on the vertical speed scale.
The pilot must keep the vertical speed of his aircraft outside these zones.
For example, if own and intruder's flight paths are horizontal and cross through each other, the intruder being at a higher flight level, if the vertical separation is sufficient, it is not necessary to modify the aircraft flight path. The positive vertical speed sector is in red to indicate that the aircraft may remain at its present level or may descend but must not climb.
If intruders are detected above and below, two red sectors are displayed leaving an uncolored zone around the zero value on the vertical speed scale advising the pilot to maintain the aircraft at its current level.
The following figure shows several examples of corrective and preventive advisories.
Preventive resolution advisories advise the pilot to avoid vertical speeds that could lead to a hazardous situation.
They are represented by one or two red sectors on the vertical speed scale.
The pilot must keep the vertical speed of his aircraft outside these zones.
For example, if own and intruder's flight paths are horizontal and cross through each other, the intruder being at a higher flight level, if the vertical separation is sufficient, it is not necessary to modify the aircraft flight path. The positive vertical speed sector is in red to indicate that the aircraft may remain at its present level or may descend but must not climb.
If intruders are detected above and below, two red sectors are displayed leaving an uncolored zone around the zero value on the vertical speed scale advising the pilot to maintain the aircraft at its current level.
The following figure shows several examples of corrective and preventive advisories.
(5) Messages annunciated on the ND
As well as intruder information, the ND also displays operating mode messages or fault data. This information is presented in the lower section of the ND (message zone):
As well as intruder information, the ND also displays operating mode messages or fault data. This information is presented in the lower section of the ND (message zone):
- TA ONLY - white - for the TA mode (automatic or manual switching)
- TCAS - red - to indicate a TCAS computer failure.
(6) Display on the PFD
A red TCAS flag appears to the left of the vertical speed scale on the PFD if the TCAS cannot deliver RA data.
A red TCAS flag appears to the left of the vertical speed scale on the PFD if the TCAS cannot deliver RA data.
(7) Display on the upper ECAM DU
- If a TCAS fault is detected, the amber warning message NAV TCAS FAULT is displayed on the upper ECAM DU.
- Selection of the TCAS STBY mode on the ATC/TCAS control unit results in the display of the TCAS STBY message (green) in the memo section of the upper ECAM DU.
H. ATC/TCAS Control Unit Operational Use
The TCAS is a cooperative system whose operating mode is very close to the ATC/Mode S transponder associated to it.
The main controls are thus grouped on the ATC/TCAS control unit and the traffic and conflict resolution information is presented on the EFIS displays.
The TCAS is a cooperative system whose operating mode is very close to the ATC/Mode S transponder associated to it.
The main controls are thus grouped on the ATC/TCAS control unit and the traffic and conflict resolution information is presented on the EFIS displays.
NOTE: - operation of the ATC/Mode S transponder is described in 34-52.
The manual operating modes of the TCAS are selected via the ATC/TCAS control unit | - the EFIS components are also described in 31-64 for the PFD |
| and 31-65 for the ND. |
(1) TCAS modes of operation
The TCAS mode of operation is selected by means of two selectors switches.
The TCAS mode of operation is selected by means of two selectors switches.
- STBY, TA and TA/RA
- THRT, ALL, ABV and BLW
(a) STBY mode
In the Standby Mode, the advisory generation and surveillance functions are inhibited. No TCAS information can be displayed on the PFDs and NDs.
The aircraft symbol and the range ring remain on the ND and vertical speed information is not displayed on the PFD.
The green TCAS STBY message is displayed in the memo section of the upper ECAM DU.
If the mode selector is set to XPDR, the TCAS also operates in standby mode.
In the Standby Mode, the advisory generation and surveillance functions are inhibited. No TCAS information can be displayed on the PFDs and NDs.
The aircraft symbol and the range ring remain on the ND and vertical speed information is not displayed on the PFD.
The green TCAS STBY message is displayed in the memo section of the upper ECAM DU.
If the mode selector is set to XPDR, the TCAS also operates in standby mode.
(b) TA mode
In this mode, intruders are displayed on the ND according to their position in the airspace. The RA type intruder symbols are converted into TA type symbols. The TCAS performs surveillance functions but does not generate any resolution advisories.
The TA ONLY message is displayed in white on the NDs in the left corner of the TCAS message area.
In this mode, intruders are displayed on the ND according to their position in the airspace. The RA type intruder symbols are converted into TA type symbols. The TCAS performs surveillance functions but does not generate any resolution advisories.
The TA ONLY message is displayed in white on the NDs in the left corner of the TCAS message area.
(c) TA/RA mode
The TCAS performs all TA mode functions and also issues preventive or corrective resolution advisories, represented in the form of colored sectors along the vertical speed scale on the PFD.
The sensitivity level is determined automatically in function of altitude.
The TCAS performs all TA mode functions and also issues preventive or corrective resolution advisories, represented in the form of colored sectors along the vertical speed scale on the PFD.
The sensitivity level is determined automatically in function of altitude.
NOTE: The final mode of operation is a combination taking into account the aircraft altitude and any instructions received from ATC Mode S ground stations. For further information refer to the priority logic definition (Ref. Para. 6.F.).
(d) THRT mode
In this mode, TCAS intruders (proximate or/and other) are displayed if a TA or a RA is already displayed.
In this mode, TCAS intruders (proximate or/and other) are displayed if a TA or a RA is already displayed.
(e) ALL mode
This selection enables display of all intruders without any conditions (TCAS intruders are displayed when detected).
This selection enables display of all intruders without any conditions (TCAS intruders are displayed when detected).
(f) ABV and BLW modes
This selection controls the above and below vertical altitude for traffic advisory:
This selection controls the above and below vertical altitude for traffic advisory:
- ABV: altitude range is set to 9900 ft above the aircraft and 2700 ft below
- BLW: altitude range is set to 9900 ft below the aircraft and 2700 ft above.
(2) ATC mode of operation
The mode of operation of the transponder is selected by a switch with three positions STBY, AUTO, ON.
The mode of operation of the transponder is selected by a switch with three positions STBY, AUTO, ON.
(a) STBY mode
When the transponder is in standby it does not transmit either squitters or replies to ground station or other aircraft interrogations.
When the transponder is in standby it does not transmit either squitters or replies to ground station or other aircraft interrogations.
(b) AUTO mode
In flight, the aircraft operates as in the ON mode: all its functions are active.
When the aircraft touches down, the landing gear ground/flight relay disables the Mode A and C replies of the selected transponder from ground station interrogations.
However the Mode S replies and squitter are not inhibited when the A/C is on the ground.
In flight, the aircraft operates as in the ON mode: all its functions are active.
When the aircraft touches down, the landing gear ground/flight relay disables the Mode A and C replies of the selected transponder from ground station interrogations.
However the Mode S replies and squitter are not inhibited when the A/C is on the ground.
(c) ON mode
The Mode S transponder operates permanently, both in flight and on the ground. It periodically transmits squitters (at 1 second intervals) to be detected by other aircraft and replies to their interrogations and those from ground stations.
This function permits, on ground, to override the inhibition of replies from interrogations in Mode A or C. It is used by the air traffic controller to check the correct operation of the aircraft Mode A or C transponder prior to takeoff.
The Mode S transponder operates permanently, both in flight and on the ground. It periodically transmits squitters (at 1 second intervals) to be detected by other aircraft and replies to their interrogations and those from ground stations.
This function permits, on ground, to override the inhibition of replies from interrogations in Mode A or C. It is used by the air traffic controller to check the correct operation of the aircraft Mode A or C transponder prior to takeoff.
(3) Altitude reporting
(a) OFF position
Selection of the ALT RPTG switch OFF position has the following results:
Selection of the ALT RPTG switch OFF position has the following results:
- the Mode S transponder continues to transmit squitters and replies to Mode A, C and S interrogations but does not report altitude.
- the TCAS switches to STBY (TCAS STBY message in green on the upper ECAM DU),
- the PFD and ND display corresponds to the standby mode.
(b) ON position
The active transponder replies to interrogations and reports its altitude. The TCAS operates in the mode selected by its logic in function of information input from the control unit, ATCRBS/MODE S ground stations and in function of aircraft altitude.
The active transponder replies to interrogations and reports its altitude. The TCAS operates in the mode selected by its logic in function of information input from the control unit, ATCRBS/MODE S ground stations and in function of aircraft altitude.
(4) Selection of system 1/2 active transponder
The 1/2 switch permits selection of the active transponder. The non-selected transponder is placed in standby.
The 1/2 switch permits selection of the active transponder. The non-selected transponder is placed in standby.
(5) Identification code in Mode A
The Mode S transponder also replies to Mode A interrogations from ground stations. A keyboard permits the pilot to set the Mode A octal code assigned to the aircraft by the ATC ground station controller and included in the transmitted replies. A window on the control unit displays this code permanently as long as the content of the digital output message complies with the displayed data.
The Mode S transponder also replies to Mode A interrogations from ground stations. A keyboard permits the pilot to set the Mode A octal code assigned to the aircraft by the ATC ground station controller and included in the transmitted replies. A window on the control unit displays this code permanently as long as the content of the digital output message complies with the displayed data.
(6) IDENT pushbutton switch
On ground station request, an addition pulse must be included in the Mode A and Mode C replies transmitted by the transponder to enable a more precise location. This operation is performed by pressing the IDENT pushbutton switch on the control unit.
On ground station request, an addition pulse must be included in the Mode A and Mode C replies transmitted by the transponder to enable a more precise location. This operation is performed by pressing the IDENT pushbutton switch on the control unit.
7. Test
A. Self-Test
A quick check of the correct operation of the TCAS installation can be performed by activating the TEST function:
A quick check of the correct operation of the TCAS installation can be performed by activating the TEST function:
- either by pressing the pushbutton switch on the front of the TCAS computer
- or through the CFDIU by applying the procedure TCAS functional test on the MCDU.
- resolution advisory characteristics (0 ft/mn advisory, up corrective advisory, don't descend, don't climb > 2000 ft/mn, rate to maintain) on label 270
- label frames 130, 131, 132 containing the data for each of the four intruders according to the following table:
-------------------------------------------------------------------------- ! INTRUDER ! TYPE ! RANGE ! REL ALT ! BEARING ! VERTICAL RATE ! ! ! ! (NM) ! (FEET) ! (DEG) ! ! !------------!--------!---------!-----------!-----------!----------------! ! 1 ! RA ! 2.00 ! +200 ! +90 !no vertical rate! ! 2 ! TA ! 2.00 ! -200 ! -90 !climbing ! ! 3 ! PROX ! 3.625 ! -1000 ! +33.75 !descending ! ! 4 ! OTHER ! 3.625 ! +1000 ! -33.75 !no vertical rate! --------------------------------------------------------------------------
(1) ND image
The ND must display the images corresponding to the four types of intruders: Other, Proximate, TA and RA.
The shapes and colors of the traffic symbols are:
The ND must display the images corresponding to the four types of intruders: Other, Proximate, TA and RA.
The shapes and colors of the traffic symbols are:
- white outlined diamond for Other traffic
- white diamond for Proximate traffic
- yellow circle for TA traffic
- red square for RA traffic
(2) PFD image
At the beginning of the test sequence, green and red sectors must appear sequentially on the vertical speed scale of the PFD. Then a resolution advisory display is shown.
At the beginning of the test sequence, green and red sectors must appear sequentially on the vertical speed scale of the PFD. Then a resolution advisory display is shown.
(3) Fault indication
At the end of the test sequence, the system generates a synthesized voice message:
In this case the NDs, the PFDs and the ECAM show the fault messages described in Para. 6.G.(5), (6) and (7).
In addition, two windows on the front of the TCAS computer display a code identifying the failed component according to the table below:
At the end of the test sequence, the system generates a synthesized voice message:
| TCAS SYSTEM TEST OK |
| if the system operates correctly or: |
| TCAS SYSTEM TEST FAIL |
| if an anomaly has been detected. |
In this case the NDs, the PFDs and the ECAM show the fault messages described in Para. 6.G.(5), (6) and (7).
In addition, two windows on the front of the TCAS computer display a code identifying the failed component according to the table below:
| ------------------------------------------------------------------------------- |
| ! CONTROL/ ! LOCATION ! FUNCTION ! |
| ! INDICATING ! ! ! |
| !------------------!-------------!--------------------------------------------! |
| ! TTR PASS ! Front panel ! Comes on to indicate TTR has passed self- ! |
| !(indicator light) ! ! test ! |
| ! TTR FAIL ! Front panel ! Comes on to indicate TTR has failed self- ! |
| !(indicator light) ! ! test ! |
| ! XPNDR ! Front panel ! Comes on to indicate transponder or data ! |
| !(indicator light) ! ! link interface failure ! |
| ! UPPER ANT ! Front panel ! Comes on to indicate upper TCAS antenna ! |
| !(indicator light) ! ! failure ! |
| ! LOWER ANT ! Front panel ! Comes on to indicate lower TCAS antenna ! |
| !(indicator light) ! ! failure ! |
| ! RAD ALT ! Front panel ! Comes on to indicate lack of radio ! |
| !(indicator light) ! ! altimeter data ! |
| ! HDNG ! Front panel ! Comes on to indicate lack of heading data ! |
| !(indicator light) ! ! ! |
| ! R/A ! Front panel ! Comes on to indicate RA indicator failure ! |
| !(indicator light) ! ! ! |
| ! T/A ! Front panel ! Comes on to indicate TA indicator failure ! |
| !(indicator light) ! ! ! |
| ! TEST (pushbutton ! Front panel ! Initiates self-test when pressed ! |
| ! switch) ! ! ! |
| ------------------------------------------------------------------------------- |
B. CFDS
(1) Introduction
The CFDS permits to present the TCAS computer faults on the Multipurpose Control and Display Units (MCDU) and on the printer.
The TCAS computer which is classified Type 1 system in the CFDS also delivers data related to fault diagnoses required for its maintenance.
It communicates with the CFDS through two ARINC 429 low speed buses.
The CFDS is characterized by two operating modes:
The CFDS permits to present the TCAS computer faults on the Multipurpose Control and Display Units (MCDU) and on the printer.
The TCAS computer which is classified Type 1 system in the CFDS also delivers data related to fault diagnoses required for its maintenance.
It communicates with the CFDS through two ARINC 429 low speed buses.
The CFDS is characterized by two operating modes:
- MENU mode
- NORMAL mode.
(2) Menu Mode
(a) SYSTEM REPORT/TEST function
This function enables access to all the electronic systems and to the TCAS computer.
The display of the SYSTEM REPORT/TEST page for the NAV components on which the TCAS is accessible is shown on the following figure.
When the line key adjacent to the TCAS indication is pressed, the CFDIU establishes the dialog and provides access to one of the eight following items:
This function enables access to all the electronic systems and to the TCAS computer.
The display of the SYSTEM REPORT/TEST page for the NAV components on which the TCAS is accessible is shown on the following figure.
When the line key adjacent to the TCAS indication is pressed, the CFDIU establishes the dialog and provides access to one of the eight following items:
- LAST LEG REPORT
- PREVIOUS LEGS REPORT
- LRU IDENTIFICATION
- GROUND SCANNING
- TROUBLE SHOOTING DATA
- CLASS 3 FAULTS
- GROUND REPORT
- DISCRETE CONFIGURATION
- TEST.
(b) LAST LEG REPORT
This report contains the computer internal and external faults recorded during the last flight.
The following tables give the lists of the fault messages which can be reported and indicate the fault class assigned to them as defined by the CFDS standard.
This report contains the computer internal and external faults recorded during the last flight.
The following tables give the lists of the fault messages which can be reported and indicate the fault class assigned to them as defined by the CFDS standard.
(3) Internal faults
The following figure shows the sequence of menus to display these messages.
| ------------------------------------------------------------------------------- |
| ! LIST OF INTERNAL FAULT MESSAGES ! |
| !-----------------------------------------------------------------------------! |
| ! ATA ! CLASS ! MESSAGE ! |
| !-----------------------------------------------------------------------------! |
| ! 344334 ! 1 ! TCAS (1SG) ! |
| ! 344311 ! 1 ! TCAS TOP ANTENNA (7SG1) ! |
| ! 344311 ! 1 ! TCAS TOP ANTENNA (7SG1) ! |
| ! ! ! COAXIAL J1 ! |
| ! 344311 ! 1 ! TCAS TOP ANTENNA (7SG1) ! |
| ! ! ! COAXIAL J2 ! |
| ! 344311 ! 1 ! TCAS TOP ANTENNA (7SG1) ! |
| ! ! ! COAXIAL J3 ! |
| ! 344311 ! 1 ! TCAS TOP ANTENNA (7SG1) ! |
| ! ! ! COAXIAL J4 ! |
| ! 344311 ! 1 ! TCAS BOT ANTENNA (7SG2) ! |
| ! 344311 ! 1 ! TCAS BOT ANTENNA (7SG2) ! |
| ! ! ! COAXIAL J1 ! |
| ! 344311 ! 1 ! TCAS BOT ANTENNA (7SG2) ! |
| ! ! ! COAXIAL J2 ! |
| ! 344311 ! 1 ! TCAS BOT ANTENNA (7SG2) ! |
| ! ! ! COAXIAL J3 ! |
| ! 344311 ! 1 ! TCAS BOT ANTENNA (7SG2) ! |
| ! ! ! COAXIAL J4 ! |
| ------------------------------------------------------------------------------- |
The following figure shows the sequence of menus to display these messages.
(4) External faults
External faults and associated messages are listed in the table below:
External faults and associated messages are listed in the table below:
| ------------------------------------------------------------------------------- |
| ! LIST OF EXTERNAL FAULT MESSAGES ! |
| !-----------------------------------------------------------------------------! |
| ! ATA ! CLASS ! MESSAGE ! |
| !-----------------------------------------------------------------------------! |
| ! 344233 ! 3 ! RA1 (2SA1)/TCAS (1SG) ! |
| ! 344233 ! 3 ! RA2 (2SA2)/TCAS (1SG) ! |
| ! 344233 ! 1 ! RA1 (2SA1)+RA2 (2SA2)/TCAS (1SG) ! |
| ! 345233 ! 3 ! ATC 1(1SH1)/TCAS (1SG) ! |
| ! 345233 ! 3 ! ATC 2(1SH2)/TCAS (1SG) ! |
| ! 345233 ! 1 ! ATC1 (1SH1)+ATC2 (1SH2)/TCAS (1SG) ! |
| ! 341234 ! 3 ! ADIRU1 (1FP1)/TCAS (1SG) ! |
| ! 240000 ! 1 ! POWER SUPPLY INTERRUPT ! |
| ! 355212 ! 1 ! ATC - TCAS CTL PNL (3SH)/TCAS (1SG) ! |
| ! 313234 ! 3 ! CFDIU (1TW)/TCAS (1SG) ! |
| ------------------------------------------------------------------------------- |
NOTE: Fault of one ATC is a class 3 fault.
Fault of both ATCs is a class 1 fault.
Fault of both ATCs is a class 1 fault.
NOTE: Fault of one radio altimeter is a class 3 fault whereas it becomes a class 1 fault when two radio altimeters are faulty.
The class 3 recorded external faults are displayed through the CLASS 3 FAULTS menu sequence as shown in the following figure. (5) PREVIOUS LEGS REPORT
The messages are identical to those shown in the Para. above but concern the last 63 flights if faults have occurred during these flights.
The messages are identical to those shown in the Para. above but concern the last 63 flights if faults have occurred during these flights.
(7) GROUND SCANNING
This function enables consultation of the TCAS computer fault recordings as defined by the Component Manufacturer.
The TCAS peripheral monitoring and internal cyclic tests are used in order to detect transient faults.
This function enables consultation of the TCAS computer fault recordings as defined by the Component Manufacturer.
The TCAS peripheral monitoring and internal cyclic tests are used in order to detect transient faults.
(8) TROUBLE SHOOTING DATA
This function allows to analyze the snapshot of the recorded fault to detect any software bug.
Two types of data are displayed:
This function allows to analyze the snapshot of the recorded fault to detect any software bug.
Two types of data are displayed:
- some correlation parameters which are the date and the UTC hour displayed in clear english,
- some snapshot data delivered in hexadecimal code or in clear message.
(9) GROUND REPORT
This function is used to present class 1, 2 or 3 internal faults when they are detected on ground. These faults differ from those displayed on the LAST LEG REPORT page.
The following figure shows examples of internal faults recorded on ground by the TCAS system.
The relevant trouble shooting data are displayed by pressing the line key adjacent to the fault indication.
This function is used to present class 1, 2 or 3 internal faults when they are detected on ground. These faults differ from those displayed on the LAST LEG REPORT page.
The following figure shows examples of internal faults recorded on ground by the TCAS system.
The relevant trouble shooting data are displayed by pressing the line key adjacent to the fault indication.
(10) DISCRETE CONFIGURATION
This function is used to present the value or the status of several discretes. A report screen for each data shows the status of the discrete at the time this menu is selected.
This function is used to present the value or the status of several discretes. A report screen for each data shows the status of the discrete at the time this menu is selected.
(11) TCAS functional test
A TCAS built-in functional test can be initiated by pressing the line key adjacent to the TEST indication on the TCAS maintenance sub-menu.
The following sequence is then generated.
A TCAS built-in functional test can be initiated by pressing the line key adjacent to the TEST indication on the TCAS maintenance sub-menu.
The following sequence is then generated.
- the test ends with the display of the following message on the MCDU:
TEST OK or TEST FAIL - at the end of the functional test the TCAS computer generates a complete sequence of synthesized voice messages.
When all the tests are completed and no fault has been detected the computer reports TCAS SYSTEM TEST OK.
If one of the test criteria has not been met, the synthesized voice message TCAS SYSTEM TEST FAIL is announced.
Remarks: - this short test permits to check rapidly, on ground, the functionality of the TCAS and particularly that of the PFD/NDs via the DMC.
In addition it enables to check correct operation of the synthesized voice message system - to date the DMCs cannot present the OTHER type intruders.
The future Full Time Display installation will call for their presentation.
(12) Software configuration follow-up
It must be possible to check correct downloading by using the CFDS/MCDU interface.
The LRU IDENTIFICATION item displays the downloaded software version reference (line SW/N).
It must be possible to check correct downloading by using the CFDS/MCDU interface.
The LRU IDENTIFICATION item displays the downloaded software version reference (line SW/N).